Gyromagnetic resonance method and apparatus for obtaining spin-spin coupling constants

ABSTRACT

Spin-spin coupling constants are obtained by exciting and detecting a decaying train of time displaced spin echo resonances of a plurality of spin-spin coupled groups of gyromagnetic resonators, such as a group of chemically shifted homonuclear atomic nuclei, within a sample under analysis. The envelope of the peak amplitude of the train of spin echo resonances decays and is modulated in accordance with the spin-spin coupling constants. The modulated envelope is Fourier analyzed to separate the individual Fourier frequency components, each component corresponding to one-half the sums and differences of the different spin-spin coupling constants, whereby a spectrum of spin-spin coupling constants is obtained which is free of magnetic field inhomogeneity effects and chemical shifts.

United States Patent 91 Freeman m1 3,753,081 Aug. 14, 1973 GYROMAGNETICRESONANCE METHOD AND APPARATUS FOR OBTAINING SPIN-SPIN COUPLINGCONSTANTS Inventor:

[75] Raymond Freeman, Menlo Park,

[1.8. CI. 324/05 R Int. Cl. G01n 27/78 Field of Search 324/0.5 R, 0.5 A,

References Cited OTHER PUBLICATIONS R. Freeman and H. D. W. Hill-HighResolution Studies of NMR Spin Echoes: J. Spectra" Journal of Chem.Physics 54(1) Jan. 1, 1971 pp. 301-313 Primary ExaminerMichael J. LynchAttorney-Harry E. Aine and Gerald M. Fisher [57] ABSTRACT Spin-spincoupling constants are obtained by exciting and detecting a decayingtrain of time displaced spin echo resonances of a plurality of spin-spincoupled groups of gyromagnetic resonators, such as a group of chemicallyshifted homonuclear atomic nuclei, within a sample under analysis. Theenvelope of the peak amplitude of the train of spin echo resonancesdecays and is modulated in accordance with the spin-spin couplingconstants. The modulated envelope is Fourier analyzed to separate theindividual Fourier frequency components, each component corresponding toone-half the sums and differences of the different spin-spin couplingconstants, whereby a spectrum of spin-spin coupling constants isobtained which is free of magnetic field inhomogeneity effects andchemical shifts.

12 Claims, 7 Drawing Figures Al? OSCILLATOR (IS A SIDEBAND TRANSMITTER LGENERATOR PULSE ['2 r5 SEOUENCER GATE 1 PROBE Q 3 1 5 [9 RECE IVER [6TIMER AND 1 ETEH'OR AY Rah-3 GATE A T0 0 CONVERTER FQURIER /TRANSFORMERPAIENIEB AUG 14 I973 SHEETIUFZ I? l M 4 [Rig FIG.2

f RF A SIDEBAND ECHO TRANSMITTER GENERATOR L r5 P SEOU R REER GATE T (MI |.osEc PROBE TIME a s H H 9 ,6 FIG.3 1 RECEIVER TIMER AND JAB CL OJABETE TOR T g F R CHO CCLZH DELAY AH I0-3 ATOD FIG.4 CONVERTER I A= TFOURIER 2 B/TRANSFORMER V FREQUENCY DISPLAY FIG 5 PATENTED NIB 14 I975ECHOES FIG.6

FIG.7

J SPECTRUM (AMX) J SPECTRUM SHEHZUFZ I/ZU -J J SPECTRUM J SPECTRUM (X)FREQUENCY (Hz) DESCRIPTION OF THE PRIOR ART l-Ieretofore, it has beenobserved that the decaying envelope of the peak amplitude of atrain ofspin echo pulses contains a modulation component of a frequencycorresponding to one-half of the spin-spin coupling constant between twospin-spin coupled chemically shifted homonuclear groups of nuclei withinthe sample. Such an'observation was made by E.L. Hahn and DE. Maxwell inthe physical Review, Vol. 88, page 1070, (1952); and by 1.6. Powles andA Hartland in the Proceedings of the Physical Society of London, Vol.77, page 273, (1961).

These prior observations were made for a relatively simple molecule,namely, one in which spin-spin coupling was present between only twogroups of gyromagnetic resonators. In such a case, there is only onespinspin coupling constant leading to only one frequency of modulationon the decaying envelope of the peak amplitude of the train ofsuccessive spin echoes. This modulation component was readily abstractedfrom a recording of the modulated envelope by merely measuring theperiod of the modulation to derive the frequency of the coupling,thereby yielding a measure of the coupling constant in terms offrequency. Such coupling constants are typically in the range from toseveral Hz for homonuclear spin-spin coupling constants.

While the teachings of the prior art demonstrated that the spin-spincoupling constant between two spinspin coupled groups of gyromagneticbodies could be extracted from the modulated envelope of the peakamplitudes of the decaying sequence of spin echoes, there was noteaching or suggestion of how to extract the coupling constants fromsuch a modulated envelope when more than one'spin-spin coupling constantwas involved. More particularly, when there is spinspin coupling betweenmore than two groups of gyromagnetic bodies, the number of spin-spincoupling constants are multiplicative rather than additive and sums anddifferences of spin-spin coupling constants are observed. For N numberof different spins N (N-l )/2 coupling constants are involved and theenvelope modulation contains N X 2 Fourier components. Moreover, eachFourier frequency component of the modulation decays at a different ratewhile the unmodulated spin echo envelope is decaying at a differentrate.

Thus, where a plurality of spin-spin coupling constants are involved, itis extermely difficult if not impossible to apply the prior art methodof visually observing the period of a particular modulation component inthe envelope and deriving from the observed period its frequency andthus the coupling constant.

It is important to ascertain the spin-spin coupling constants in orderto facilitate and simplify analysis of complex gyromagnetic resonancespectra. For example, in the gyromagnetic resonance spectrum of acomplex molecule, nuclear resonance lines may be broadened due tomagnetic field inhomogeneities and lines split by spin-spin coupling andshifted due to chemical shifts within the spectrum. In such cases,analysis of the resultant spectra can be substantially simplified bydetermining the spin-spin coupling constants and assigning thedetermined spin-spin line splittings in the complex spectra to simplifyidentification of the conventionally obtained spectrum which includeschemical shift effects.

SUMMARY OF THE PRESENT INVENTION neity effects. r

In another feature of the present invention, at least one of a pluralityof spin-spin coupled groups of gyromagnetic resonators is decoupled fromthe other spins in an excited train of spin echo resonances by excitingforced continuous resonant precession of the group to be decoupled,whereby the group which is excited into forced precession is spin-spindecoupled from the remaining spin-spin coupled groups to substantiallysimplify the modulation of the decaying envelope of peak amplitude ofthe train of spin echoes.

In another feature of the present invention, the modulation of thedecaying envelope of a train of spin echoes is substantially simplifiedby selectively detecting spin echo resonance within a band of resonantfrequencies which excludes the resonance frequency of at least one ofthe excited groups of spin-spin coupled resonators, whereby at least onespin-spin coupling constant is eliminated from the spin-spin couplingmodulation of said spin echo envelope.

In another feature of the present invention, the decaying envelope ofthe peak spin echo amplitude for a train of spin echo resonances isfrequency analyzed by Fourier transforming the modulation of theenvelope from the time domain to the frequency domain to obtain theseparate frequency components of the modulation and thus the separatespin-spin coupling constants.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram ofa gyromagnetic resonance spectrometer incorporating features of thepresent invention,

FIG. 2 is a composite timing diagram depicting the operating cycles fortheRF transmitter, spin echoes as received'in the receiver and detector,and sampling- DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now toFIG. 1 there is shown a gyromagnetic resonance spin echo spectrometer lincorporating features of the present invention. The spectrometer 1includes a probe 2 for containing a sample of matter to be investigatedwhich is inserted within an intense uniform polarizing magnetic fieldproduced between poles 3 of a powerful electromagnet or permanentmagnet. In a typical example, the magnetic field between the poles 3 isbetween 10,000 and 50,000 gauss.

The probe 2 contains a radio frequency transmitter coil, not shown, forapplying a radio frequency magnetic field to the sample at an angle tothe polarizing magnetic field for exciting gyromagnetic resonance of thegyromagnetic resonators within the sample. The radio frequency energy issupplied to the transmitter coil from a radio frequency transmitter 4via the intermediary of a gate 5 for pulsing of radio frequency energyto the probe 2 in accordance with a desired pulse sequence, such as thesequence shown in the upper trace of FIG. 2.

The probe 2 also contains a receiver coil, not shown, which-is orientedorthogonally to the transmitter coil for picking up radio frequencyresonance signals emanating from the sample within the probe. Theresonance signals picked up by the receiver coils are fed to the inputof areceiver and detector 6 wherein they are amplified and detected andthence fed to the input of an analog to digital converter 7 via theintermediary of a sampling gate 8.

The sequence of transmitter pulses supplied to the probe 2 via the gate5 and depicted in the upper trace of FIG. 2 are sequenced andproportioned in amplitude and duration to excite a train of spin echoresonances of the gyromagnetic resonators within the sample underanalysis. A particularly suitable pulse sequence is that as taught byl-I.Y. Carr et al. in Physical Review, Vol. 94, page 630, (1954). Moreparticularly, this sequence consists of radio frequency pulses, the frequency of which are chosen to be tuned to the Larrnor or gyromagneticresonance frequency of the particular resonators to be excited intoresonance, such as protons which are the nuclei of hydrogen atoms in aliquid molecular sample material under analysis such as3-bromothiophene-2-aldehyde.

The first pulse of the sequence, identified as 90, has an amplitude andduration to tip the spins of the gyromagnetic bodies by approximately 90from the direction of the polarizing magnetic field between the poles 3.The magnitude of the radio frequency magnetic field is selected toexcite uniformly all the various chemically shifted groups ofgyromagnetic bodies, such as protons, within the sample under analysis.

In a typical example of a proton magnetic resonance, the chemical shiftspectrum may extend for a bandwidth of approximately 1,000 Hz. In such acase, the

v radio frequency magnetic field should have sufficient intensity toprovide substantially constant resonance excitation over the expectedbandwidth of the spectrum. Thus, in a typical example, the bandwidthbetween half power points of the RF exciting field should beapproximately 10 KHz which would require the radio frequency magneticfield to have an intensity of approximately 2.5 gauss within the sample.

The duration of the first radio frequency pulse applied to the sample isselected such that the gyromagnetic resonators will be tipped orprecessed to an orientation approximately 90 to the direction of thepolarizing magnetic field. The transmitter pulse is then abruptlyterminated and the gyromagnetic resonators are allowed to freely precessabout the direction of the polarizing field.

During their precession, the gyromagnetic resonators lose phasecoherence due to a number -of different effects including magneticshielding due to the presence of the electrons around the nucleus of thehydrogen atoms and their particular site in the molecule, due to theinhomogeneity'in the magnetic field, and due to spin-spin couplingconstants between the hydrogen nucleior gyromagnetic resonators disposedin different sites in the molecule. 1

After some predetermined interval of time 1', which may be ratherarbitrarily selected and which may comprise for example 0.5 second butsuch time should be selected to satisfy the following relation; 1' 1/21r 1d. 11 9i? I l ..IPE llFE PQIYYF in?QBSlQfIhQ pulse and the beginningof the next or refocusing pulse identified 180 and 8 is the smallestchemical shift frequency in the spectrum under analysis.

Thus, after the inverval r, a second radio frequency pulse (180) isgated by gate 5 to the probe 2 which is substantially identical to thefirst 90 pulse but which is twice the duration of the first pulse tocause the precessing magnetic moments or resonators to reverse theirdirection of precession. Such a pulse is called a refocusing pulsebecause it causes the phase differences in the precessing bodies to berefocused to a phase coincidence or reinforcement at a period 1' afterthe end of the refocusing pulse.

Such a'reinforced pulse is called a spin echo and is shown by the secondsignal trace in FIG. 2, and identified as ECHO. The positive half of theenvelope of the spin echo is shown on the echo trace but it is to beunderstood that the echo signal is precessing at substantially theLamior or gyromagnetic resonance frequency at or near the radiofrequency of transmitter 4.

Thus, after a period 1 following the peak of the spin echo signal, asecond (180) refocusing pulse is applied to the sample to cause theprecessing magnetic moments to flip by another 180 and refocus at a time21' following the peak of the first echo signal.

The train radio frequency transmitter pulses applied to the sample hasthe sequence 90 -1802 r-l80-2 'r-180 The spin echoes have a sequencewith a peak amplitude occurring at n(2'r) following termination of thefirst transmitter pulse, where n can have any integer value. However, itwill be noted that the peak amplitude of each successive echo signal hasa reduced amplitude in accordance with a decay; with the time constantT, of the decay being the transverse relaxation time for the particulargyromagnetic resonator being examined. Thus, in a time of approximately2T, the amplitude of the spin echoes decays into the noise level and thesequence is terminated.

The sampling gate 8 is timed by means of a timer 9 and delay 11 such asto sample the peak amplitude of the envelope of the successive echoes inaccordance with the sampling sequence identified as GATE in FIG. 2. Thetimer 9, in a typical example, compris'esthe timer portion of adedicated general purpose digital computer such as a Varian Data Machinemodel 6201' digital computer commercially available from VarianAssociates of Palo Alto, Calif.

The timer 9 also serves to actuate a pulse sequencer 12 for deriving thesequence of transmitter pulses as indicated above with regard to thetransmitter sequence of FIG. 2. The output of the pulse sequencer 12 isfed to control gate 5 for gating the transmitter pulses to the probe 2.The timer '9, which also synchronizes the sampling gate 8., causes thesampling gate 8 to sample the received spin echo envelope in accordancewith the peak amplitude of each of the spin echo envelopes, namely, at apoint 1 after termination of each of the 180 refocusing pulses. Due todelays in the receiver and detector 6, delay 11 causes opening of thesampling gate 8 to be delayed by approximately 1 millisecond relative to7 after the refocusing pulses to compensate for the delays in thereceiver and detector 6.

The output of the sampling gate 8 is fed to an analog to-digitalconverter 7 wherein the sampled peak amplitude of the successive echoesis converted into a digital input which is fed to the input of a Fouriertransformer 13. The Fourier transformer 13 may comprise any one of anumber of different devices.

In a preferred embodiment, the Fourier transformer 13 includes thememory of a 620i digital computer and the sampled output from the gate 8is fed and stored in successive channels of the memory of the computer.In a typical example, there may be as many as 150 or more pulse echoesgenerated by one initial 90 transmitter pulse such echoes beingsuccessively refocused by the sequence of 180 refocusing pulses untilsuch time as the echo amplitude decays into the noise.

The successively sampled amplitudes of the echoes are stored insuccessive channels of the Fourier transformer l3 and upon completion ofthe sampling sequence, i.e., upon the echoes decaying into the noiselevel, the transformer 13 performs a Fourier transformation on thestored digital information to produce and separate the individualFourier components of the sampled spin echo envelope. The Fouriercomponents are separated and displayed on a display 14 such as an X-Yrecorder where the amplitude of the individual Fourier components areplotted as a function of frequency.

The Fourier transformer 13 may comprise, for example, a Fouriertransform computer for Fourier transforming the envelope of the spinecho resonances. Such a Fourier transform computer is disclosed andclaimed in US. Pat. No. 3,475,680 issued 28 Oct. 1969. As analternative, an analog Fourier transformer may be employed which recordsthe modulated spin echo envelope of the spin echo resonances in anoptical recorder such as on a photographic film which is then processedand passed through an optical spectrograph as an optical diffractiongrating which is analyzed by a photo electric densitometer to producethe separate Fourier components. As an alternative, the spin echoenvelope signal is recorded on a magnetic tapeand repetitively playedbaclt and heterodyned with the frequency of a variable frequencyoscillator. The beat difference frequencies are fed through a narrowband filter and detected to separate the Fourier components. Such Fourier analyzers are disclosed and claimed in US. Pat. No. 3,287,629issued Nov. 22, 1966.

The preferred Fourier transformer embodiment of the present inventionutilizes a programmed Fourier transform digital computer such as the620i as disclosed in US. Pat. No. 3,475,680. The computer is programmedin accordance with the :table of algorithm provided in an article titledThe Fast Fourier Transform Algorithm of J.W. Cooley and J.W. Tukey,appearing in Mathematical Computations, Vol. 19, page 297, (1965) andmodified to include the modifications suggested by G.D. Bergland in theCommunications of the Association for Computing Machinery, Vol. 11, page703, (1968).

Referring now to FIG. 3, there is shown a typicalnuclear magneticresonance spectrum for the protons of the molecule depicted in'FlG. 3and obtained, for example, by scanning through the resonance spectrumwith a swept polarizing magnetic field in a conventional gyromagneticresonance spectrometer, not shown. In such a molecule, the protons aredivided into two different groups, namely, group H and H These twogroups of protons are chemically shifted and spin-spin coupled. Thespectrum for the protons is as shown in FIG. 3 and comprises twochemically shifted lines, shifted by frequency or magnetic field Hequivalent to 8 In addition, each of the chemically shifted lines issplit due to the spin-spin coupling constant J between the group of Hprotons and group of H protons and the width of each of the chemicallyshifted lines is proportional to the gradients of thepolarizing magneticfield AH.

In a molecule having only two spin-spin coupled groups of chemicallyshifted nuclei there is only one spin-spin coupling constant. For thesample case of FIG. 3, the coupling constant J M may be readilyseparated and identified by the spectrometer 1 of FIG. 1. It turns outthat the decaying envelope of peak echo amplitude for the train of spinechoes :is modulated by the spin coupling constnt J Fourier analysis ofthe mod-ulation of the peak spin echo envelope separates the spin-spincoupling constant. More particularly the spinspin coupling constantspectrum, hereinafter sometimes referred to as J spectrum, for thesimple case of only two groups of spin-spin coupled resonators is asshown in FIG. 4 and has only one spectral component of a frequency, onan absolute scale, equal to is the frequency splitting due to thespin-spin coupling constant.

Referring now to FIG. 5, there isshown a typical envelope of peak spinecho amplitude ,as modulated by two widely different spin-spin couplingconstants. In the particular trace of FIG. 5, it is @obvious that thereare at least two coupling constants one at a relatively low frequencyand one at a relatively high frequency. However, as the moleculebecomesrmore complicated and additional resonator groups become coupleddue to spin-spin coupling constants, themodulation of the peak spin echoenvelope becomes much more complex such as that shown in FIG. 6 for themolecule of FIG. 6.

Referring now to FIG. 6, separation of the individual Fourier componentsof the modulation on the peak spin echo resonance envelope becomes muchmore difficult if not impossible without special Fourier analyzers, suchas a Fourier transform computer 13 or an analog spectrum analyzer, asaforcdescribed.

For example, referring now to FIG. 7, there is shown in the spectrumidentified J spectrum (AMX), the spectrum of coupling constants for themolecule of FIG. 6 and depicting the spin-spin coupling spectrum for theproton groups I-I I-l and H The J spectrum AMX is obtained at the outputof the display 14 in the spectrometer of FIG. 1. This spectrum isrelatively complex and it is moderately difficult to assign theindividual lines of the spectrum to a particular coupling constant ofthe spin coupled A, X, and M groups. However, these coupling constantscan be more easily as-' certained by selective detection of resonance ofeach of the groups.

More particularly, in the spectrometer of FIG. 1, the receiver anddetector 6 is tuned to detect excited resonance of the chemicallyshifted group H, to the exclusion of the resonances of the other groups.In such a case, the A group exhibits first order spin coupling to thetwo other groups of nuclei, namely, the I-I and H, groups that are bothsubject to the influence of the RF pulses. The only points on thedecaying train of spin echoes which are unaffected by chemical shift andfield inhomogeneity effects are the spin echo peaks (at the times 2m),and it is at these points that the signal is sampled by the samplinggate 8. These echo signals are sampled at a frequency F, equal to 1/21-Hz. It follows from the sampling theorem that only frequency componentsbelow Fs/Z l-Iz are recoverable after the sampling process, higherfrequency components being folded back into the range to Fs I-Iz/2. Hz.These discrete points measured over a total period T seconds define acomplex waveform,

As schematically indicated in FIG. 6, the discrete Fourier transform ofthis wave is readily calculated by a standard Fourier transform programin a conventional Fourier transform computer, as above described. Theresult of the Fourier transformation from the time domain to thefrequency domain of the waveform of equation I is a spectrum extendingfrom 0 to F,/2 Hz with sampling points every 1/T l-Iz, containing tworesonance responses at (J J )/2 and (J J d/2 Each line has a Lorentzshape with a full width T essentially uneffected by magnetinhomogeneity; Such a spectrum is shown by the J spectrum identified bytrace (A) of FIG. 7 and is defined as a partial J spectrum due toselective detection of only the A group resonance.

The decomposition of the complex spectrum (AMX) into two modulationcomponents and a spin-spin relaxation time is thus achieved in a veryconvenient fashion. Note that the spin-spin relaxation time T, isdefined at the half amplitude points of the spectral line, as indicatedin FIG. 4.

Note that the spin echo modulation due to spin-spin coupling to severaldifferent resonant nuclei is multiplicative rather than additive, justas in spin-spin splitting in conventional first order nuclear magneticresonance spectra. This is the reason why sums and differences of spincoupling constants are observed in the J spectrum rather than thecoupling constants themselves.

lution of the pairs of simultaneous equations represented by each set ofspectra.

The partial J spectra, as shown in FIG. 7, are useful for determiningand assigning the individual coupling constants to the complex Jspectra. Moreparticularly, the individual partial J spectra each definea set of simultaneously equations that may be readily solved for theindividual spin coupling constants. For the particular example shown inFIG. 7, the solution of the simultaneous equations result in thefollowing spin coupling constants:

J $3.47 10.19 Hz J $1.77 20.05 Hz J 1:0.87 $0.06 Hz In addition toselective detection for obtaining partial J spectra, coupling constantsmay be deleted from complex J spectra by spin-spin decoupling. Morespecifically, an excited resonance group to be decoupled is selectivelyradiated with continuous radio frequency energy at its Larmor orgyromagnetic resonance frequency. In the spectrometer of FIG. 1, thegroup of gyromagnetic resonators to be decoupled, such as the H group ofprotons, is radiated withradio frequency en-- ergy at its Larmorfrequency by modulating a sample of the radio frequency transmittersignal in a sideband generator 16 with an audio frequency signal derivedfrom an audio frequency oscillator 17, such that one of the generatedsidebands is at the Larmor frequency of the group of gyromagneticresonators to be decoupled. A switch 18 is closed to allow the selectedsideband to be applied to the transmitter coil for applying the radiofrequency energy to the group to be decoupled. If desired, additionalaudio frequency oscillators may be added and additional sidebandsgenerated for decoupling one or more groups of gyromagnetic spin coupledgyromagnetic resonators for further simplifying the resultant partial Jspectra.

Although the invention, thusfar described, has been specificallydirected to determining the spinspin coupling constants betweenhomonuclear groups of resonators this is not a requirement as the methodand apparatus may be employed for determining heteronuclear spin-spincoupling constants such as the coupling constant between fluorine andhydrogen in molecules containing both hydrogen and fluorine. In such acase, the radio frequency transmitter 4 will transmit radio frequencyenergy at both the Larmor frequency of the protons and the Larmorfrequency of the fluorine atoms for simultaneously exciting spin echoresonances of both heteronuclear groups of resonators.

In a preferred embodiment of the spectrometer 1 of FIG. 1, thepolarizing magnetic field is preferably stabilized by a conventionalfield-frequency control, not shown, which holds constant the ratio ofthe frequency of the resonance exciting radio frequency field to theintensity of the polarizing magnetic field. Typically thefield-frequency control locks either the frequency of the transmitter 4or the polarizing field intensity to a gyromagnetic resonance linewithin the sample or a control group of gyromagnetic resonators immersedin the same polarizing field as the sample under analysis.

What is claimed is:

1. In a method for obtaining spin-spin coupling gyromagnetic resonancedata from a sample of matter immersed in a polarizing magnetic field thesteps of, exciting and detecting a sequence of time displaced spin echoresonances of a plurality of spin-spin coupled groups of gyromagneticresonators within the sample, said sequence of detected spin echoresonances having an envelope of echo peak amplitude which is modulatedin accordance with a plurality of spin-spin coupling constants betweenat least three groups of spinspin coupled gyromagnetic resonators, andanalyzing said modulation of said envelope of spin echo peak amplitudeto separate at least one of the Fourier components thereof, wherebyspin-spin coupling constant gyromagnetic resonance spectral data isobtained which is substantially free of magnetic field inhomogeneityeffects.

2. The method of claim 1 wherein the step of analyzing said modulationof said envelope of echo peak amplitude includes the step of, excitingforced precession of at least one of said spin-spin coupled groups ofgyromagnetic resonators to decouple the spin of the forced precessionalgroup of gyromagnetic resonators from at least two other groups ofspin-spin coupled resonators to remove at least one of the Fouriercomponents from said modulation of said envelope to simplifysaidanalysis of said modulation of said envelope.

3. The method of claim 1 wherein the step of detecting said excitedsequence of spin echo resonances includes the step of, selectivelydetecting said excited spin echo resonances within a band ofgyromagnetic resonance frequencies which excludes at least thegyromagnetic resonance frequency of at least one of said excited groupsof spin-spin coupled gyromagnetic resonators, whereby at least onespin-spin constant Fourier component is eliminated from said spin-spincoupling modulation of said envelope of spin echo amplitude.

4. The method of claim 1 wherein the step of analyzing said modulationof said envelope of spin echo peak amplitude includes the step of,Fourier transforming said modulation from the time domain to thefrequency domain.

5. The method of claim 1 wherein said at least three groups ofgyromagnetic resonators are separate chemically shifted groups ofhomonuclei.

6. In an apparatus for obtaining spin-spin coupling gyromagneticresonance data from a sample of matter immersed in a polarizing magneticfield, means for exciting a sequence of time displaced spin echoresonances of a plurality of spin-spin coupled gyromagnetic couplingconstant between at least a plurality of said excited groups ofspin-spin coupled gyromagnetic resonators, and means for analyzing saiddetected modulation of said envelope of spin echo peak amplitude toseparate at least one of the Fourier components therefrom, wherebyspin-spin coupling; constant gyromagnetic resonance spectral data isobtained which is substantially free of magnetic field inhomogeneityeffects.

7. The apparatus of claim 6 wherein said means for detecting saidsequence of spin echoes and for producing an output in accordance withthe envelope of detected peak echo amplitude includes, means forsampling the echo amplitude of said sequency of spin echos substantiallyonly at the peak amplitude of each respective echo.

8. The apparatus of claim 6 wherein said sample includes at least threespin-spin coupled groups of gyromagnetic resonators to define at leastthree spin-spin coupling constants, and including means for excitingforced precession of at least one of said spin-spin coupled groups ofgyromagnetic resonators during the sequence of spin echo resonance of atleast the other two groups of resonators to decouple the spins of theforced precessional group of gyromagnetic resonators from at least thetwo other groups of spin-spin coupled resonators to remove at least oneof the spin-spin coupling Fourier components from said modulation ofsaid envelope to simplfy analysis of said modulation of said envelope.

9. The apparatus of claim 6 wherein said means for detecting saidsequence of excited spin echo resonances includes, means for selectivelydetecting said excited spin echo resonances within a band ofgyromagnetic resonance frequencies which excludes at least thegyromagnetic resonance frequency of at least one of said excited groupsof spin-spin coupled gyromagnetic resonators, whereby at least onespin-spin coupling constant Fourier component is eliminated from saidspin-spin coupling modulation of said envelope of peak echo amplitude.

10. The apparatus of claim 6 wherein said means for analyzing saiddetected modulation of said envelope of spin echo peak amplitude toseparate at least one of the Fourier spin-spin coupling constantcomponents therefrom includes, means for Fourier transforming saidmodulation from the time domain to the frequency domain.

11. The apparatus of claim 10 wherein said Fourier transforming meansincludes a programmed digital computer.

12. The apparatus of claim 6 wherein said spin-spin coupled groups ofgyromagnetic resonators are separate chemically shifted homonucleargroups of atomic nuclei.

1. In a method for obtaining spin-spin coupling gyromagnetic resonancedata from a sample of matter immersed in a polarizing magnetic field thesteps of, exciting and detecting a sequence of time displaced spin echoresonances of a plurality of spin-spin coupled groups of gyromagneticresonators within the sample, said sequence of detected spin echoresonances having an envelope of echo peak amplitude which is modulatedin accordance with a plurality of spin-spin coupling constants betweenat least three groups of spin-spin coupled gyromagnetic resonators, andanalyzing said modulation of said envelope of spin echo peak amplitudeto separate at least one of the Fourier components thereof, wherebyspin-spin coupling constant gyromagnetic resonance spectral data isobtained which is substantially free of magnetic field inhomogeneityeffects.
 2. The method of claim 1 wherein the step of analyzing saidmodulation of said envelope of echo peak amplitude includes the step of,exciting forced precession of at least one of said spin-spin coupledgroups of gyromagnetic resonators to decouple the spin of the forcedprecessional group of gyromagnetic resonators from at least two othergroups of spin-spin coupled resonators to remove at least one of theFourier components from said modulation of said envelope to simplifysaid analysis of said modulation of said envelope.
 3. The method ofclaim 1 wherein the step of detecting said excited sequence of spin echoresonances includes the step of, selectively detecting said excited spinecho resonances within a band of gyromagnetic resonance frequencieswhich excludes at least the gyromagnetic resonance frequency of at leastone of said excited groups of spin-spin coupled gyromagnetic resonators,whereby at least one spin-spin constant Fourier component is eliminatedfrom said spin-spin coupling modulation of said envelope of spin echoamplitude.
 4. The method of claim 1 wherein the step of analyzing saidmodulation of said envelope of spin echo peak amplitude includes thestep of, Fourier transforming said modulation from the time domain tothe frequency domain.
 5. The method of claim 1 wherein said at leastthree groups of gyromagnetic resonators are separate chemically shiftedgroups of homonuclei.
 6. In an apparatus for obtaining spin-spincoupling gyromagnetic resonance data from a sample of matter immersed ina polarizing magnetic field, means for exciting a sequencE of timedisplaced spin echo resonances of a plurality of spin-spin coupledgyromagnetic resonators within said sample, means for detecting saidsequence of excited spin echo resonances and for producing an output inaccordance with the envelope of detected peak echo amplitude, suchenvelope being modulated in accordance with at least one spin-spincoupling constant between at least a plurality of said excited groups ofspin-spin coupled gyromagnetic resonators, and means for analyzing saiddetected modulation of said envelope of spin echo peak amplitude toseparate at least one of the Fourier components therefrom, wherebyspin-spin coupling constant gyromagnetic resonance spectral data isobtained which is substantially free of magnetic field inhomogeneityeffects.
 7. The apparatus of claim 6 wherein said means for detectingsaid sequence of spin echoes and for producing an output in accordancewith the envelope of detected peak echo amplitude includes, means forsampling the echo amplitude of said sequency of spin echos substantiallyonly at the peak amplitude of each respective echo.
 8. The apparatus ofclaim 6 wherein said sample includes at least three spin-spin coupledgroups of gyromagnetic resonators to define at least three spin-spincoupling constants, and including means for exciting forced precessionof at least one of said spin-spin coupled groups of gyromagneticresonators during the sequence of spin echo resonance of at least theother two groups of resonators to decouple the spins of the forcedprecessional group of gyromagnetic resonators from at least the twoother groups of spin-spin coupled resonators to remove at least one ofthe spin-spin coupling Fourier components from said modulation of saidenvelope to simplfy analysis of said modulation of said envelope.
 9. Theapparatus of claim 6 wherein said means for detecting said sequence ofexcited spin echo resonances includes, means for selectively detectingsaid excited spin echo resonances within a band of gyromagneticresonance frequencies which excludes at least the gyromagnetic resonancefrequency of at least one of said excited groups of spin-spin coupledgyromagnetic resonators, whereby at least one spin-spin couplingconstant Fourier component is eliminated from said spin-spin couplingmodulation of said envelope of peak echo amplitude.
 10. The apparatus ofclaim 6 wherein said means for analyzing said detected modulation ofsaid envelope of spin echo peak amplitude to separate at least one ofthe Fourier spin-spin coupling constant components therefrom includes,means for Fourier transforming said modulation from the time domain tothe frequency domain.
 11. The apparatus of claim 10 wherein said Fouriertransforming means includes a programmed digital computer.
 12. Theapparatus of claim 6 wherein said spin-spin coupled groups ofgyromagnetic resonators are separate chemically shifted homonucleargroups of atomic nuclei.